Sulawesi is the largest island in the Wallacea biogeographic region. Though it has a unique mountainous geology only a few studies have investigated how its vegetation change with elevation. Additionally, none of these studies address forest changes across lowland elevation gradients. To overcome this information gap, this study examines how the structure and composition of tropical tree communities change across a lowland elevation gradient in the Tangkoko-Duasudara Nature Reserve, North Sulawesi, Indonesia. Thirty-three 40 m x 40 m plots were placed at 100 m elevation increments from 0 to 1000 m above sea level (a.s.l. for short). We measured height and DBH for trees and identified them to species. We found that trees in middle elevations were shorter than their lowland and highland counterparts and may be caused by the middle slopes being more prone to disturbance or to droughty or infertile soil. Alpha diversity was not significantly correlated with elevation but beta diversity changed dramatically. Analyses using NMDS showed decrease in beta diversity as elevations increased, indicating significant turnover and change in species composition across elevation. This study provides new insight on patterns of forest structure and composition in an understudied tropical ecosystem. My results can inform policies and management practices so as to create effective conservation plans.